A materials handling vehicle is known in the prior art comprising a base unit including a power source and a mast assembly. A fork carriage assembly is coupled to the mast assembly for vertical movement relative to the power source with at least one cylinder effecting vertical movement of the carriage assembly. A hydraulic system is coupled to the cylinder for supplying a pressurized fluid to the cylinder, and includes an ON/OFF blocking valve positioned in a manifold for preventing the carriage assembly from drifting downwardly when raised via the cylinder to a desired vertical position relative to the power source. A metering valve, also positioned in the manifold, defines the rate at which pressurized fluid is metered to the cylinder to raise the carriage assembly and metered from the cylinder to lower the carriage assembly. A velocity fuse, i.e., a mechanical valve, is positioned in a base of the cylinder to prevent an unintended descent of the carriage assembly in excess of approximately 120 feet/minute. The velocity fuse has a fixed setpoint such that it is closed and stops fluid flow at the cylinder when the carriage assembly downward speed exceeds about 120 feet/minute. Hence, such fuses will not permit controlled downward movement of a carriage assembly at a speed in excess of about 120 feet/minute. However, it would be desirable to allow an intended descent of a carriage assembly in a controlled manner at a speed in excess of 120 feet/minute to improve productivity.
It is noted that when a velocity fuse closes, it closes very quickly resulting in a hydraulic fluid pressure spike occurring within the cylinder. Such a pressure spike can cause the cylinder to bow, buckle or otherwise deform. It would be desirable to reduce such pressure spikes. It would also be desirable to eliminate the velocity fuse so as to remove cost from the vehicle.
It is also known in the prior art to use flow control valves in place of velocity fuses. Those valves are designed to limit the flow of hydraulic fluid from a lift support cylinder such that a carriage assembly is prevented from moving downwardly at a speed in excess of about 120 feet/minute. Because such valves are mechanical, they too will not permit controlled downward movement of a carriage assembly at a speed in excess of about 120 feet/minute.